There is no way to purchase credits separately on the Hobby plan. Your $5/month subscription already covers $5 of resource usage each billing cycle, and if your usage exceeds that, you're automatically billed for the overage at the end of the cycle. Prepaid credits are not available - Railway uses post-paid billing, so you simply keep using your services and pay for any usage beyond the included $5 on your next invoice.

thank you for your answer, but my services are down and as far as I know it is because of the extra usage that is not covered. maybe I'm doing something wrong?, I find it fine to pay for the extra usage in the next bill.

Your workspace is active and all five of your services currently show successful deployments, so they are not stopped due to billing or usage limits. On the Hobby plan, usage beyond the included $5 is simply billed as overage at the end of your billing cycle (your current cycle usage is about $6.34, so you'll owe roughly $1.34 extra). If your services seem unresponsive, the cause is not credit-related. You can review your usage and configure a hard limit at your workspace usage page.

thanks so much for the clarification